Patent GB-688462-A (GB)
Title: Improvements relating to proportional integral electrical controlling apparatus
Inventor: BOUNDY JAMES REILLY
IPC: G05D9/12
Filed: 1949-02-16
Abstract: 688,462. Fluid-pressure servomotor-control systems. EVERSHED & VIGNOLES, Ltd. March 14, 1949 [Feb. 16, 1949], No. 4306/49. Class 135 [Also in Group XXXV] In a system for automatically controlling a variable in which a controlling electric current has components respectively proportional to the magnitude and the integral of any deviation, the latter component is derived from the former by causing the former to control, differentially, the illumination of a pair of photocells in a bridge circuit associated with a condenser in a thermionic valve circuit. As shown in a system for maintaining constant oil level in reservoir R despite variable inflow through a pipe P, variation in level is detected by a float F which moves contacts 16, 17 relative to a contact 19 to apply to a resistancecondenser network 20, 22, a positive or negative voltage from a potential divider 14 according to the sense of the variation. The voltage across condenser 22 which then becomes increasingly positive or negative is applied to the grid-cathode circuit of a triode 7 the anode current of which varies until contact 19 is re-centralised by a moving coil 5 carrying the anode current. Thereafter, the anode current remains at a value corresponding to the changed level and energises a moving coil 38 to upset the balance of a pivoted lever 93 biased by a spring 90, to move a blade relative to an air nozzle 104 controlling a pneumatic valve 97 which in turn controls an outlet valve V to restore the oil to the desired level. Moving coil 38 by controlling through a pivoted lever 93 the position of the blade relative to nozzle 104 controls the pressure developed in a bellows 98 from an air pressure source 94 through a pipe 102 of fine bore. Bellows 98 controls the position of valves 97, 103 which differentially control supply and vent orifices associated with a pipe 96 the pressure in which, acting on a diaphragm 87 against a spring 89, controls the position of valve V so that it takes up a position dependent on the energisation of coil 38. Mechanical follow up is provided by a pivoted lever 91 connected to valve V and acting through spring 90 on lever 93 in opposition to coil 38. To produce an integral control component the anode current of valve 7 energises a moving coil'108 to operate a shutter 110 differentially controlling light from a source 111 to two photocells 112, 113 connected in a bridge circuit the output of which upon unbalance is applied through a resistancecondenser integrating circuit 119, 120 to the grid circuit of a triode valve 122. A differential component is also introduced into the grid circuit of valve 122 by applying the voltage across a resistor 134 in the anode circuit of valve 7 to a resistancecondenser differentiating circuit 124, 135. A current dependent on the sum of integral and differential components is thus produced in the anode circuit of valve 122 and this current contributes to the energisation of coil 38 so that the components are introduced into the control system. The magnitudes of the three components of control may be adjusted by variable resistors 134, 136, 137. Enclosure of the control apparatus in flameproof casings is referred to. Specifications 265,012, [Class 38 (iii)], 589,423, [Group XXXVIII], and 688,516, are referred to.